The present invention relates in general to gas turbine combustors and more specifically to premixed combustors having improved flashback margin.
In a combustion system for a gas turbine, fuel and compressed air are mixed together and ignited to produce hot combustion gases that drive a turbine and produce thrust or drive a generator for producing electricity. In an effort to reduce pollution levels, government agencies have introduced new regulations requiring gas turbine engines to reduce emitted levels of emissions, including carbon monoxide and oxides of nitrogen (NOx). A common type of combustion, employed to comply with these new emissions requirements, is premix combustion, where fuel and compressed air are mixed together prior to ignition to form as homogeneous a mixture as possible and burning this mixture to produce lower emissions. While premixing fuel and compressed air prior to combustion has its advantages in terms of emissions, it also has certain disadvantages such as combustion instabilities. More specifically, premix combustion stability issues of concern include combustion dynamics and flashbacks.
In order to achieve the lowest possible emissions through premixed combustion, without the use of a catalyst, it is desirable to provide a fuel-lean mixture to the combustor. However, the richer the fuel content in a combustor, the more stable the flame and combustion process. Therefore, fuel-lean mixtures tend to be more unstable given the lesser fuel content for a given amount of air. As a result, when fuel-lean mixtures are burned they tend to produce greater pressure fluctuations due to the unstable flame.
Another area of concern with gas turbine combustors and their stability is with respect to flashback and the migration of a flame upstream from the desired location. Flashback typically occurs when the flame migrates upstream from the combustion chamber through the combustor boundary layer, or the location along the combustor walls where flow velocity is the lowest, to the premixing sections of the combustor. A flame can move up through the low flow velocity region in the boundary layer if pressure fluctuations occurring within the combustor allow the flamefront to move upstream and a combustible mixture is present in the boundary layer to sustain the flame. In operation, if a combustor flashes back, the flashback can cause significant damage to combustor hardware if going undetected, possibly requiring premature replacement or repair. Furthermore, a flashback can affect engine performance by unexpectedly reducing power output. Flashback margin is typically provided by ensuring the bulk fuel/air velocity in the premixer substantially exceeds the flame speed of the fuel/air mixture, thus ensuring that the flame cannot propagate upstream into the premix chamber. However, for high flame speed fuels, establishing flashback margin, can require extremely high velocities, which in turn results in high combustion pressure drop and hence deterioration of engine efficiency.
In order to provide a more stable premix combustion process over the prior art, while also obtaining the lowest emissions possible, it is necessary to provide a means for purging the boundary layer of the combustor, such that no combustion reaction can take place in the boundary layer.